Introduction

Type 2 diabetes is a growing epidemic, with approximately 20.8 million persons with diagnosed and undiagnosed type 2 diabetes in the United States (National Institutes of Health 2007). Diabetes-related costs in the United States are staggering and result from mortality, permanent disability, and lost productivity (Hogan 2003). Improved glycemic management, as measured by reduced glycosylated hemoglobin (A1c), can minimize the risk of diabetes complications (UKPDS 1998), lower health care costs, and increase workplace productivity (Shetty 2005, Stephens 2006, Testa 1998, Tunceli 2007, Von 2005, Wagner 2001).

Adherence to medications plays a critical role in the achievement and maintenance of glycemic control. Adherence to diabetes medication regimens has been associated with improved glycemic control and reduced health care costs (Krapek 2004, Lawrence 2006, Lee 2006, Schectman 2002, Sokol 2005, Wagner 2001). Patients’ out-of-pocket expense (cost share) has been identified as one of several factors that can influence adherence to prescribed medications (Briesacher 2007, Chernew 2008, Cole 2006, Ellis 2004, Gibson 2005, Gibson 2006a, Gibson 2006b, Zeber 2007). Research suggests that 32 percent of older adults take less medication than prescribed in order to avoid costs (Soumerai 2006).

The extent to which patient cost share affects adherence and, therefore, health outcomes is important to understand, given recent insurance trends. In response to escalating pharmaceutical costs, pharmacy benefit design has evolved to increase the portion of medication expense borne by beneficiaries. For patients enrolled in employer-sponsored health plans, copayments for prescription drugs increased significantly between 2000 and 2006, rising from $13–$24 to $17–$38 (Kaiser Family Foundation & Health Research and Educational Trust 2007). Such changes in pharmacy benefit design may have unintended effects on patients’ adherence to medications for chronic conditions. The strategy of shifting costs to patients in an attempt to combat escalating health care expenditures and patient cost-sharing as an impediment to care are topics of widespread debate (Braithwaite 2007).

For patients with diabetes, data that link patient cost share with adherence to diabetes medications and clinical outcomes are sparse. This study was undertaken to evaluate the associations between patient medication cost share and (1) adherence to oral diabetes medications and (2) glycemic control.